One way to engage with some of the other levels is to find a qualified instructor in Gachirinkan meditation, which is among the most openly taught practices of Shingon Buddhism and is also found in the Tendai tradition.. This practice was almost certainly part of the Founder's early Shingon education, since there is documentary evidence of his initiation into "higher" teachings for which it is a prerequisite. It was also on the somewhat eclectic menu of practices that were part of Oomoto-kyo. Among the buildings destroyed in the Second Oomoto Incident was the Gekkyu-den, which seems to have been a site of Gachirinkan practice.

While this, and other practices in that tradition, are often advertised as promoting immediate tangible benefits, those immediate benefits are generally regarded as byproducts of the practices, rather than the purpose of the practices, which has a broader scope than the simple (mind you, simple doesn't mean easy) development of physical power..

The tricky bit is that a qualified instructor -- which means "fully ordained priest authorized to teach" -- who feels that the applicant is only seeking the immediate benefits of the practice may decline to teach him or her.

My take is that with this, as with some other practices in his repertoire, the Founder didn't teach any of his students precisely because he honored his vows; anybody who wants the material has to go to the same place he went, in several senses of the word "place."

Well, again we're sort of back to vagaries. What other benefits? I know some other benefits, but I can functionally relate them back as usual to the functional core, even though they may sound unrelated to the casual listener.

Insofar as Ueshiba not teaching parts of his own art because he was keeping his vows, makes you wonder why he even bothered, doesn't it?